1. Field of the Invention
The present invention relates to an optical amplifier for use in an optical transmission system, and more particularly to an optical direct amplifier device and a bidirectionally pumped optical direct amplifier device.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows a basic circuit of a conventional optical direct amplifier device. As shown in FIG. 1, the conventional optical direct amplifier device has a rare-earth-doped optical fiber 1a serving as an optical amplification fiber, a WDM (Wavelength Division Multiplexing) coupler 2a, and an optical isolator 4a which are successively connected in the order named from an upstream signal input terminal to an upstream signal output terminal. The conventional optical direct amplifier device also has a rare-earth-doped optical fiber 1b serving as an optical amplification fiber, a WDM coupler 2b, and an optical isolator 4b which are successively connected in the order named from a downstream signal input terminal to a downstream signal output terminal. The WDM couplers 2a, 2b have other input terminals connected respectively to two output terminals of an optical multiplexer/demultiplexer 4 whose two input terminals are connected respectively to laser diodes 6a, 6b. The laser diodes 6a, 6b are connected respectively to control circuits 7a, 7b which control the laser diodes 6a, 6b to emit light at a constant output level.
The conventional optical direct amplifier device has a maximum amplified output level corresponding to the maximum pumping light emission level of a single laser diode. The conventional optical direct amplifier cannot meet demands for amplified output powers higher than the maximum amplified output level.
The conventional optical direct amplifier device is also problematic in that it will be completely out of operation if the two laser diodes 6a, 6b suffer a failure and their light emission is turned off.
FIG. 2 of the of the accompanying drawings shows a basic circuit of a conventional bidirectionally pumped optical direct amplifier device. As shown in FIG. 2, the conventional bidirectionally pumped optical direct amplifier device has a WDM coupler 2a, a rare-earth-doped optical fiber 1a, a WDM coupler 3a, and an optical isolator 4a which are successively connected in the order named from an upstream signal input terminal to an upstream signal output terminal. The WDM couplers 2a, 3a have other input terminals connected respectively to laser diodes 6a, 6c that are connected respectively to control circuits 7a, 7c. The above arrangement serves as an upstream optical direct amplifier. The conventional bidirectionally pumped optical direct amplifier device also has a downstream optical direct amplifier which is identical to the upstream optical direct amplifier.
The conventional bidirectionally pumped optical direct amplifier device is disadvantageous in that the upstream or downstream optical direct amplifier will be completely out of operation if the two laser diodes 6a, 6c or 6b, 6c suffer a failure and their light emission is turned off.